I personally, do not leave canned food out any longer than an hour...it is, after all wet food, and with the potential for attracting flies, and other things, and being a breeding ground for bacteria...
While it's true, a healthy cats stomach acid, and a healthy gut, can knock out, and withstand some pretty nasty bacteria, that would have us going to the hospital, a cat that's young, or has a compromised immune system, or is old, can get sick from bad food...
So I just don't think it's worth risking to leave food out for hours on end...
Sharon